Intermittent Fasting & Exercise: Is It Safe?
Intermittent fasting (IF) and the ketogenic diet can go hand-in-hand. Both diets help your body by limiting glucose, although they do so in different ways. Keto limits glucose by adhering to high-fat and low-carb meals, while IF reduces glucose through systematic fasting.
If you’re on keto and IF, is it safe to continue fasting after a workout? In this article, we’ll cover the basics of IF and how to safely continue fasting after a workout.
Intermittent Fasting 101
Intermittent fasting is referred to as simply IF, but you might come across the term written as intermittent energy restriction or even time-restricted eating. Regardless of what you call it, IF refers to a regular, timed cycle of eating and then fasting.
There are several different types of IF, and they are all categorized by fasting windows.
- 16/8: A fasting cycle in which your fasting window is 16 hours, and your window for eating is 8 hours. This is the most popular cycle and the easiest to maintain, especially if you skip breakfast. Many people eat from 12 noon until 8 pm with the remaining 16 hours reserved for fasting.
- 18/6: During this fast, you’ll eat only between a 6-hour window, often between noon and 6 pm.
- 20/4: Like the previous two schedules, this one splits the time between fasting and eating. In this case, you’ll only eat during a four-hour window each day.
- 24-hour fasting: During this IF schedule, you’ll fast for an entire day. For this schedule, you’ll choose one day per week to fast, and the remaining six days are normal eating schedules.
- 5:2: During this type of fasting schedule, you’ll alternate how many calories you eat. For example, two days per week, you might eat a very low-calorie diet (approximately 500 calories) while the remaining five days are 2000-calorie days.
Is It Safe to Exercise While You’re Fasting?
While IF is safe and effective for weight loss and improved brain function, it is safe to mix with exercise? Research shows that fasting before a workout is not only safe but, has numerous benefits. Namely, exercising on an empty stomach can help burn more fat, reduce your energy intake, and increase fat oxidation. 
Exercising on an empty stomach can be safe, but there are a few caveats.
Benefits of exercising while fasting
- You may burn more fat.
- You can continue your IF schedule without having to skip your workout.
Cons of exercising while fasting
- Your performance may decrease if you run out of energy stores.
- You risk dehydration and electrolyte depletion, if not supplementing properly.
Should You Worry about Electrolytes While Working Out?
Whether you are working out during a fast or not, it’s important to keep your electrolytes balanced. Electrolytes are minerals – salt, potassium, magnesium, and calcium – that send chemical messages through your body. Electrolytes are essential. They promote a healthy heart – including heartbeat regulation and regulated blood pressure – but they also help with muscle contractions, body temperature regulation, and even proper nerve function in your body.
If you’re working out, you should pay particular attention to your electrolytes. That’s because when you fast, your body can lose electrolytes if you’re not consuming foods like dark leafy greens, bone broth, and dairy products.
Symptoms of Electrolyte Depletion
If your electrolytes are not balanced, it isn’t a good idea to continue working out. You might suspect you have electrolyte depletion if you experience:
- A racing heart
- Muscle cramps
Protocol for Exercising During a Fast
The most important thing you can do while exercising during a fast is to listen to your body. If you experience dizziness or weakness – or feel faint – it’s a good idea to back off and reevaluate. Low blood sugar, dehydration, and electrolyte imbalances can cause these symptoms.
In order to avoid dizziness and weakness, follow this protocol for exercising while fasting:
- Prioritize hydration. You may not be eating, but you need to consume plenty of water. Without it, you run the real risk of becoming dehydrated – and that’s a fast track to a terrible workout.
- Don’t forget about your electrolytes. Drink an electrolyte-containing drink after your workout. Just be sure there are no added sweeteners in your drink. You can also add sea salt to your meals to make sure you’re getting enough salt
- Time your workout, if possible. If you exercise near the end of your fast, you’ll be able to replenish your body with nutrients during your recovery period.
- Choose the right type of activity. If you’re on a 24-hour fast, choose a lighter activity like walking or yoga. A more rigorous workout during a 24-hour fast can increase your risk of fatigue and dehydration.
- Choose wisely for your post-workout meal. Whether you eat 45 minutes or an hour after your workout, be sure to fuel your body with what it needs. For example, if you are lifting weights, your body needs protein to help recover and build muscles. Your post-workout meal should include plenty of protein.
What’s the Verdict? Is Fasting After Exercise Safe?
While it can be safe to continue your fast after working out, it is important to stay vigilant when it comes to hydration and electrolyte imbalances. What type of workout you choose and how you time your workout can also play a big role in the safety of exercising while fasting.
Do you fast after exercise? Share your experience with us!
- Willcox, Bradley J, and Donald C Willcox. “Caloric Restriction, Caloric Restriction Mimetics, and Healthy Aging in Okinawa: Controversies and Clinical Implications.” Current Opinion in Clinical Nutrition and Metabolic Care, U.S. National Library of Medicine, Jan. 2014.
- Longo, Valter D, and Mark P Mattson. “Fasting: Molecular Mechanisms and Clinical Applications.” Cell Metabolism, U.S. National Library of Medicine, 4 Feb. 2014.
- Bachman, Jessica L, et al. “Exercising in the Fasted State Reduced 24-Hour Energy Intake in Active Male Adults.” Journal of Nutrition and Metabolism, Hindawi Publishing Corporation, 2016.
Kathryn Trudeau is a content writer and educational professional whose work centers on health and wellness and organic living. She has a passion for nutrition and fitness. She loves matcha, running, and crocheting.